My 2 cents, your mileage might vary and whatnot, but in my experience I've never managed to mess up developing of negatives, except on 2 occasions (one was due to Kodak's inability to get their development times for HC110 right), and the other time I was trying to push Tri-X to 12500 ASA in Rodinal, which was doomed to fail anyhow), so don't worry so much. Basically, all you need is developer, fixer and water, all the other stuff is twist that you add as you go along, except maybe distilled water if your tap water is rich in calcium or otherwise undrinkable.

About temperature, it's only really critical at the developing stage, and as long as you are within a degree of the "ideal" temp, you'll be fine. If your development temperature is more than a couple degrees from room temp, I usually start a degree north or south of my ideal temp (i.e. if your room temp is over your dev temp, start a degree cold, and vice versa). After that, you only need to be within 5 degrees of ideal, 20-25C maybe. Of course you shouldn't get cavalier with the temperature just because you can, as precise as possible is always good, but don't worry so much about it either that you forget to have fun.

About stop baths, wash aids and other great things; you do not need them! I stopped in water for a minute and rinsed in water with washing soap for 4 years before getting anything fancier, and I can't say that I see any difference in results after adding all these frills. The only really great thing that I'd say is mandatory is 2 bath fixing, basically you mix 2 batches of fixer, and fix 2,5 min in the first, and 2.5 min in the second, and when your first bath gets full of silver and stops working, you get rid of it, mix a new batch, and use the old second bath as first bath. This way, you're always fixing optimally, as well as minimizing the use of fixer.

To your credit though, you have a lot better understanding of what you're doing going in than what did when I started out, I just bought some random chems and an old leaky tank from my local photo store and went to town.